Bug 951: weaken pointer from JSObject to cache_entry

Bug 951: weaken pointer from JSObject to cache_entry

The SpiderMonkey scripting module handles the "pre-format-html" event
by constructing a JSObject for the struct cache_entry and then calling
elinks.preformat_html(cache_entry, view_state) if such a function
exists.  The problem with this was that each such JSObject kept the
struct cache_entry locked until SpiderMonkey garbage-collected the
JSObject, even if the user had not defined an elinks.preformat_html
function and the JSObject was thus never needed at all.  To work
around that, the SpiderMonkey scripting module ran a garbage
collection whenever the user told ELinks to flush caches.

Remove the SpiderMonkey scripting module's use of object_lock and
object_unlock on struct cache_entry, and instead make the pointers
weak so that ELinks can free the cache_entry whenever it wants even if
a JSObject is pointing to it.  Each cache_entry now has a pointer back
to the JSObject; done_cache_entry calls smjs_detach_cache_entry_object,
which follows the pointer and detaches the cache_entry and the JSObject
from each other.

This commit does not yet remove the workaround mentioned above.